Incremental position measuring devices are used in the automation technology sector and, in particular, in machine tools for the purpose of measuring positional changes of mobile parts. For example, incremental rotary encoders measure rotatory movements, e.g., of rotating shafts. Incremental linear encoders, on the other hand, measure linear displacements of machine parts that are disposed so as to allow them to shift position relative to one another.
In conventional incremental position measuring devices, a graduation track, which is made up of code elements disposed at regular intervals, is scanned by a detector unit. A wide variety of physical scanning principles may be used for this purpose, e.g., optical, magnetic, inductive, or capacitive scanning principles. Preferably, the detector signals resulting from the scanning are largely sinusoidal at a uniform movement (constant speed, or constant rate of rotation), and the positional information may be obtained by counting the signal periods covered or additionally, if greater resolution is required, by subdividing the signal periods into a number of angular segments (interpolation). An item of directional information is able to be obtained when two detector signals are generated during the scanning that have a phase shift from each other, such as a phase shift of 90°. In order to provide an absolute reference point for the inherently relative position measurement of incremental position measuring devices, a reference pulse is frequently generated at at least one position. For this purpose, a suitable graduation structure may be disposed on a separate graduation track, which is likewise scanned by the detector unit.
The detector signals obtained by the detector unit are processed in a signal processing unit and adapted in accordance with a specification of an output interface. For example, one conventional interface for incremental position measuring devices requires a peak-to-peak value of 1V for the incremental signals.
In addition to such analog interfaces, there are also interfaces that output digital incremental signals. In this case, digital, i.e., square-wave, incremental signals are generated in the signal processing unit from the analog detector signals. Here, too, it applies that two digital incremental signals that are phase-shifted relative to each other are required for a position measurement as a function of a direction of movement.
Depending on the interface, the reference pulse is also output in an analog or digital manner.
The transmission of the (analog or digital) incremental signals as well as the reference pulse to a subsequent electronics takes place via high-quality multi-core cables that are shielded in the majority of cases. It may be carried out both with reference to mass and in a differential manner. Since quite long distances frequently have to be covered between the subsequent electronics and the position measuring devices, the cables represent a cost factor that should not be underestimated in the project development of a system. Because the number of required cores in the cable also has an effect on the price, there is a constant endeavor to keep the number of cores in the cable to a minimum.
Diametrically opposed to this endeavor is the requirement to also generate even further information in the position measuring device, in addition to the positional information, such as a status report. However, a transmission of such a status report to the subsequent electronics usually requires additional cores in the cable.
German Published Patent Application No. 10 2006 012 074 describes a position measuring device in which the status of a monitoring device is signaled by a change in the signal amplitudes of the analog position signals. However, since the amplitude of the position signals has a direct effect on the position evaluation in the subsequent electronics, the subsequent electronics may interpret this as a failure of the position measuring device, which may thus lead to a standstill of the system in which the position measuring device is operated.